A study of heating and cooling of the ISM in NGC 1097 with Herschel-PACS and Spitzer-IRS

NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar, and an active nucleus. We present a detailed study of the spatial variation of the far-infrared (FIR) [CII]158 mu m and [OI]63 mu m lines and mid-infrared H-2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star-forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([CII]158 mu m+[OI]63 mu m)/PAH in the ring about 50% lower than in Enuc N and S. The average 11.3/7.7 mu m PAH ratio is also lower in the ring, which may suggest a larger fraction of ionized PAHs, but no clear correlation with [CII]158 mu m/PAH(5.5-14 mu m) is found. PAHs in the ring are responsible for a factor of two more [CII]158 mu m and [OI]63 mu m emission per unit mass than PAHs in the Enuc S. spectral energy distribution (SED) modeling indicates that at most 25% of the FIR power in the ring and Enuc S can come from high-intensity photodissociation regions (PDRs), in which case G(0) similar to 10(2.3) and n(H) similar to 10(3.5) cm(-3) in the ring. For these values of G(0) and n(H), PDR models cannot reproduce the observed H-2 emission. Much of the H-2 emission in the starburst ring could come from warm regions in the diffuse interstellar medium that are heated by turbulent dissipation or shocks